A distillation method is not commonly used for separating a fluid mixture into its respective constituent components.
However, because HFC-125 and HFC-143a have such close standard boiling points of -48.5.degree. C. and -47.2.degree. C. and because the specific volatility of HFC-143a relative to HFC-125 is close to 1, a mixed fluid comprising HFC-125 and HFC-143a is known to form an azeotropic composition. Accordingly, it is very difficult to separate these components using a common distillation method alone.
Also, in the case of CFC-115 having a standard boiling point of -38.7.degree. C., its specific volatility relative to HFC-125 is also close to 1. Accordingly, a mixed fluid comprising HFC-125 and CFC-115 is know to form an azeotropic composition and is therefore very difficult to separate using a general distillation method.
To cope with these problems, an extraction distillation method can be applied, in which an extracting reagent is added to a fluid mixture forming an azeotropic composition as a third component having a standard boiling point different from the standard boiling points of the other components.
With respect to the separation of a mixed fluid having an azeotropic composition comprising HFC-125 and HFC-143a hitherto proposed, U.S. Pat. No. 3,732,150 discloses an azeotropic distillation method which comprises adding ammonia to a mixed fluid to form an azeotropic comprising HFC-143a and ammonia to thereby allow for the separation of HFC-125.
Furthermore, the present inventors have proposed in JP-A-9-12487 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") an extraction distillation method using at least one extracting reagent selected from carbon chlorides and chlorinated hydrocarbons each having 1 and 2 carbon atoms.
With respect to the separation of an azeotropic mixed fluid having an azeotropic composition comprising HFC-125 and CFC-115 hitherto proposed, U.S. Pat. No. 5,087,329 discloses an extraction distillation method using a carbon fluoride having from 1 to 4 carbon atoms or a hydrogen and/or chlorine adduct thereof.
Furthermore, the present inventors have proposed in JP-A-7-133240 an extraction distillation method as applied to a mixed fluid comprising HFC-125 and CFC-115 using an extracting agent selected from paraffinic hydrocarbons, alcohols, ethers, esters and ketones each having a standard boiling point of from -10.degree. C. to 100.degree. C. Also, the present inventors have proposed in JP-A-8-143486 an extraction distillation method using an extracting agent selected from carbon chlorides and chlorinated hydrocarbons each having 1 or 2 carbon atoms.
However, the extracting reagents used in these conventional techniques for separating a mixed fluid comprising HFC-125 and HFC-143a are not as effective as desired. Moreover, ammonia used as an azeotropic agent is toxic and dangerous to handle.
Furthermore, no publication hitherto describes a technique for separating a mixed fluid comprising HFC-125, HFC-143a and CFC-115.
As a result of investigations on the extracting reagent for separating a mixed fluid comprising HFC-125 and HFC-143a or for separating a mixed fluid comprising HFC-125, HFC-143a and CFC-115, the present inventors have accomplished the present invention.